The present invention relates to semiconductor lasers. More specifically, the present invention relates to semiconductor lasers having associated electronic components integrally formed therewith.
Semiconductor lasers are important devices used in a variety of applications including printing, scanning, communications, etc. Semiconductor lasers generally fall into two categories: edge-emitting and vertical cavity surface emitting (VCSEL). Each of these types of devices are well known. In an edge emitting semiconductor laser structure, a number of layers are deposited onto a substrate. Following deposition, the edges of the structure are cleaved to form partially transmissive mirrors. One or more of the deposited layers forms an optical cavity, bound at its edges by the mirrors. Lasing occurs within the cavity between the mirrors, and the laser beam exits at one or both of the edges of the laser structure in a direction parallel to the plane of the layers.
Surface emitting lasers are similar in concept, but differ in that the laser beam is emitted orthogonal to the plane of the active layer(s). The mirrors are above and below the optical cavity, as opposed to at each edge of the cavity. For certain applications, a surface emitting laser provides advantages over an edge emitting laser. For example, 2-dimensional arrays of vertical cavity lasers may be produced in wafer form, whereas edge emitting lasers typically must be mechanically jointed to form such arrays. Also, surface emitting lasers typically emit circularly symmetric Gaussian beams, as compared to highly eccentric elliptical beams of edge emitting lasers. Accordingly, today there is much interest and development centered around surface emitting lasers.
Associated with any semiconductor laser are numerous electronic components. For example, the power of a laser is typically controlled by the drive current applied to its electrodes; and one or more electronic components such as transistors, capacitors, diodes, etc. forming drive circuitry may be employed to control the drive current. As another example, components such as transistors are often employed in addressing circuitry for addressing individual lasers in arrays of such devices.
More specifically, in arrays of lasers, it is desirable to be able to independently address each laser. This becomes problematic when dealing with large arrays of such lasers. In such large arrays, the small size and large density of the electrodes to which connection must be made increase the complexity of making connections. Furthermore, the need to produce small size arrays limits the surface area which addressing connections and circuitry are permitted to occupy. To produce such arrays within a practical cost structure, the addressing circuitry and scheme must be relatively simple. Finally, the addressing circuitry and scheme must support rapid addressing of each laser.
While there are many examples of addressing circuitry and schemes in the art, efforts to date have not been successful in producing VCSELs having integrated (i.e., formed either as part of the process of forming the VCSEL or formed above the VCSEL as part of subsequent processing) associated electronic component structures. Rather, addressing circuitry, as well as driver circuitry and other related components, have been built external to the laser structure itself, then interconnected for operation.
For example, FIG. 1 is an illustration of a laser structure and separately connected voltage source 10. Laser 12 is formed on a substrate 14, typically GaAs. A number of thin layers are first deposited to form a lower mirror region 16, an n-type layer 18 is formed on lower mirror 16, an intrinsic active layer 20 is formed on n-type layer 18, a p-type layer 22 is formed on intrinsic active layer 20, and an upper mirror layer 24 is formed on p-type layer 22. Typically, a metal, n-material electrode 26 is formed below the substrate 14, and a metal, p-material electrode 28 is formed above the upper mirror region 18. Electrode 28 is typically annular in planform, so as to maximize surface contact yet minimize interference with the laser beam B.sub.1 generated by the structure.
A voltage is then applied to electrode 28 from external voltage supply 30 and addressing circuitry 32. Electrode 26 is typically connected to ground potential. The current through the laser 12 results in the generation of laser beam B.sub.1. The requirement of external voltage supply, addressing circuitry, and potentially other electronic components associated with laser 12 limits the ability to reduce size, cost, component complexity, etc., and increase speed, efficiency, etc.
In addition, in many laser systems it is necessary to measure and control the power of the beam emitted by the laser. For example, it is necessary in many applications to provide a constant, predetermined beam power, which requires compensation for the laser's temperature, aging, etc. Beam power detection generally involves interposing a detector in a laser beam path. In the case of certain edge emitting lasers, this may be accomplished by detecting one of two beams. That is, where an edge emitting laser is of the type having two beam emissions, one from each edge (facets), one beam is referred to as a forward emission beam and the other as a rear emission beam. The forward emission beam will generally be of a higher power than the rear emission beam. Hence, the forward emission beam is generally the operable beam performing the desired function, such as writing to a photoreceptor, pulsing encoded signals to a transmission line, cutting material, etc., while the rear emission beam is often not used. However, the ratio of the power of the forward emission beam to the power of the rear emission beam can be measured. Thus, by placing a detector in the path of the rear emission beam, and by employing the aforementioned power ratio, the power of the forward emission beam may be determined.
This approach has limited utility for surface emitting laser structures, for several reasons. Typically, surface emitting laser structures include a gallium arsenide (GaAs) substrate, which is opaque for wavelengths shorter than 870 nm. Thus, for most applications, the substrate will be opaque, and the laser structure will be capable of producing only a single, surface emitted laser beam. Second, in general it is desirable to provide as high a beam power as possible, so it is a design goal to produce single beam laser structures.
An existing approach to incorporating a detector into a single beam surface emitting laser (assumed to be single beam herein, unless otherwise stated) is to form the detector in the laser structure. That is, additional layers would be epitaxially grown above the laser structure, but of the same material as the laser structure, which would be appropriately patterned and/or doped, and interconnected to form a detector. The detector is generally coaxial with the laser beam, and relies on partial absorption of the beam to create electron-hole pairs which are detected by methods otherwise known in the art.
The upper detector electrode will have an inside diameter d, which is at its smallest equal to the diameter of the laser beam generated by the underlying laser. In operation, the detector converts photon energy from the laser beam to electron-hole pairs, which migrate to respective electrodes. Beam power is thus measured by measuring the extent of electron-hole pair generation (i.e., current generated in the detector). The speed of the detector is measured by the speed at which the electrons or holes travel to their respective electrodes. Those electrons or holes generated at the center of the annular detector electrode must travel a distance equal to at least d/2. This is a relatively large distance, and results in relatively slow detection.
Third, the detector is essentially a p-i-n photodiode. The detector's p- and n-type layers are formed of GaAs, a material opaque to the laser beam when the combined thickness of the layers is 1000 .ANG. or greater (again, assuming a laser wavelength shorter than 879 nm). Typically, a design point for absorption of the beam by the detector is around 5% of the beam's energy. However, if the layers are too thin, they will be too transparent, and not absorb sufficient photon energy to effectively serve as a detector. Thus, to obtain the desired performance from the detector, it is necessary to very precisely control the thicknesses of the p- and n-type layers, which adds to the cost and complexity of manufacture.
Whether circuitry or detector, it should be possible to tailor the thickness of the layers formed above the VCSEL to "tune" the interference of the reflected portions of the laser beam. This would provide improved efficiency and performance of the laser. However, to accomplish this, some flexibility to adjust layer thicknesses is required, which the constraints of the GaAs layers cannot provide.
In this regard, it becomes necessary to effect modifications to the laser itself to account for the additional layers at the VCSEL's surface. For example, the only way to compensate for destructive reflections from such additional layers is to adjust the thicknesses of the various laser structure layers, such as the thickness of the lower mirror layer, part of which serves as the lower mirror below the optical cavity. This constrains optimizing the laser structure design for peak laser performance.
Furthermore, there is no teaching in the art of an integrated VCSEL, sensor, and associated electronic components. According to all heretofore known techniques, such integration would significantly increase the complexity and cost of manufacturing the laser structure. In addition, the size and position limitations for the various contacts, due to their being opaque, mean that an extensive network of interconnections would be required, limiting the compaction of laser structures desired of arrays of such structures.
Consequently, the currently accepted approach for connecting laser structures with associated electronic components is to connect such structures to external circuitry. This implies a sacrifice of speed, compactness, and system complexity which the present invention strives to overcome.
In fact conventional electronics used in conjunction with VCSELs are virtually exclusively formed of single crystal silicon. However, single crystal electronics must be formed over a single crystal substrate, and thus the forming such electronics on or together with a VCSEL is very difficult if not impossible.